Differentiation of Forms of Extractable Iron and Aluminum in Soils

The approximate differentiation of crystalline and amorphous iron oxide in soils by extraction methods can be extended to a distinction between organic-complexed and amorphous inorganic Fe. Data for synthetic Fe-fulvic acid complexes, X-amorphous hydrous Fe oxide, crystalline Fe oxides and soils show that the Fe extracted by 0.1M Na4P2O7 is largely organic Fe. The difference between oxalate- and pyrophosphate-extractable Fe gives a measure of amorphous inorganic Fe, and dithionite- minus oxalate-extractable Fe provides an estimate of more or less crystalline Fe-oxides. These extractants are less useful in distinguishing forms of Al in soils. Spodic horizons contained much more pyrophosphate-extractable Fe and Al than other soil materials. A criterion for spodic horizons could be based upon pyrophosphate-extractable Al, Fe, and possibly C. The rusty mottles of several Aquepts and Aqualfs and some placic horizons low in organic matter contained goethite as well as amorphous inorganic Fe. Hematite occurred in some reddish-brown parent materials having a high ratio of dithionite- to oxalate-extractable Fe, but mottles formed in these soils contained secondary goethite.